The present invention relates to a radiation-polymerizable mixture which comprises,
(a) a compound which has at least two terminal ethylenically unsaturated groups and which forms a cross-linked polymer by means of addition chain polymerization initiated by free radicals,
(b) a polymeric binder, and
(c) a radiation-activatable polymerization initiator.
The mixture is preferably used as a photoresist material which can be transferred dry and used for producing solder masks.
It is known to produce solder masks on printed circuit boards by the photoresist technique. As in the production of printed circuits, thermoplastic photopolymerizable layers are laminated onto the printed circuit board by means of pressure and heat and are hardened in imagewise fashion by exposure in those areas where the printed circuit board is to be covered. The solder mask is obtained by washing away the unexposed layer areas.
Materials of this type are described, for example, in German Offenlegungsschrift No. 2,747,947. The materials comprise photopolymerizable layers which contain a certain amount of a bonded halogen to improve their flame-resistance. European Patent Application No. 15,004 describes a similar material which can be developed dry by mechanically separating exposed and unexposed layer areas ("peel-apart process"). Finally, European Patent Application No. 2,040 describes a light-hardenable material for the same purpose, in which the light-sensitive compounds are identified as light-sensitive epoxy resins, but are not described in more detail.
Photopolymerizable mixtures known and used for producing photoresist masks are transferred dry with heat onto the printed circuit board to be masked. Therefore, the materials must necessarily be thermoplastic. However, for use as solder masks, this thermoplasticity is disadvantageous since the masks must withstand temperatures above 200.degree. C. without decomposing and without melting or significant softening. Admittedly, the photopolymer layer is hardened to a considerable extent in the exposed areas by cross-linking polymerization, and this light-hardening can be further enhanced in a known way by re-exposure of the developed image stencil. However, the basic tendency of all known photopolymerizable layers to soften when heated remains.
In accordance with German Offenlegungsschrift No. 3,114,931 (equivalent to U.S. Ser. No. 366,379 filed Apr. 7, 1982 now U.S. Pat. No. 4,485,166) a bis-epoxy compound is added to the photopolymerizable mixture used for producing the solder mask, and the light-hardened image stencil is post-cured by heat treatment after developing in order to overcome the above-stated disadvantage. This process produces highly heat-resistant solder masks. However, the photopolymerizable mixture produced in this way has the disadvantage that its shelf life is shorter than that of an otherwise identical mixture which does not contain any epoxy compound.
European Patent Application No. 73,444 discloses similar mixtures which in the unexposed state have an improved shelf life. These mixtures contain compounds which are thermally cross-linkable with the binder, with the polymerization product of the ethylenically unsaturated compound, and/or with themselves. Preference is given to compounds in which the cross-linking groups are epoxy groups or groups of the formula EQU --CH.sub.2 --O--R,
wherein R denotes a hydrogen atom or a lower alkyl, acyl or hydroxy-alkyl group, and the groups --CH.sub.2 OR are bonded to a nitrogen atom of a low molecular weight, open-chain or cyclic acid amide or to an aromatic carbon atom of a compound which is capable of undergoing a condensation reaction with formaldehyde.
However, the mixtures described in the above publication still possess two significant drawbacks. Under particular soldering conditions, threads or pellets of the solder alloy adhere to the surface of the solder mask after wave-soldering. This occurrence, which probably is due to the softening of the layer, may possibly lead to short circuits.
Furthermore, in the fully cured state, the mixtures according to European Patent Application No. 73,444 possess an unsatisfactory resistance to solvents. As a consequence thereof, softening of the solder masks may occur during the cleaning of the soldered printed circuit boards. In addition, the dyestuff is partially dissolved out of the layer if the solvents, such as, for example, ethanol, are allowed to interact with the layer over a prolonged period.